A successor communication system to W-CDMA and HSDPA, i.e., Long Term Evolution (LTE), is currently being discussed by 3GPP, a standardization group for W-CDMA. In LTE, orthogonal frequency division multiplexing (OFDM) is to be used as a downlink radio access method and single-carrier frequency division multiple access (SC-FDMA) is to be used as an uplink radio access method (see, for example, 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006).
In OFDM, a frequency band is divided into multiple narrow frequency bands (subcarriers) and data are transmitted on the subcarriers. The subcarriers are densely arranged along the frequency axis such that they partly overlap each other but do not interfere with each other. This approach enables high-speed transmission and improves frequency efficiency.
In SC-FDMA, a frequency band is divided into multiple frequency bands and the frequency bands are allocated to different terminals for transmission in order to reduce interference between the terminals. Also, SC-FDMA reduces variation of the transmission power and therefore makes it possible to reduce power consumption of terminals and to achieve wide coverage.
In OFDM employed in LTE, two types of cyclic prefixes (CP), a long CP and a short CP having different lengths, are provided to reduce the influence of inter-symbol interference caused by a delayed signal. For example, the long CP is used in a cell with a large cell radius or to transmit a multimedia broadcast multicast service (MBMS) signal, and the short CP is used in a cell with a small cell radius. When the long CP is used, the number of OFDM symbols in a slot becomes six; and when the short CP is used, the number of OFDM symbols in a slot becomes seven.
Generally, when a mobile station is powered on, in the standby mode, in communications, or in the intermittent reception mode in a radio communication system employing W-CDMA or LTE, the mobile station has to find a cell proving good radio communication quality for the mobile station based on, for example, a synchronization channel. In other words, the mobile station searches for a cell to be connected via a radio link. Therefore, this process is called “cell search”. A cell search method is generally determined based on time necessary for a cell search and the processing load of the mobile station in the cell search. In other words, it is necessary to determine a cell search method so as to reduce the time necessary for a cell search and to reduce the processing load of the mobile station in the cell search.
In W-CDMA, two types of synchronization channels, a primary SCH (P-SCH) and a secondary SCH (S-SCH), are used for the cell search. Also in LTE, it is being discussed to employ the two types of synchronization channels P-SCH and S-SCH.
In a cell search method being discussed, a P-SCH including one sequence and an S-SCH including plural sequences are transmitted once in 5 ms (R1-062990, Outcome of cell search drafting session). In this method, the P-SCH is used to identify a downlink reception timing from each cell, and the S-SCH transmitted in the same subframe is used to determine a reception frame timing and to obtain cell-specific information such as a cell ID or a cell group (group ID). For demodulation and decoding of the S-SCH, a channel estimate obtained based on the P-SCH may generally be used. When cell IDs are grouped, a cell ID of the corresponding cell is found from cell IDs belonging to the obtained group ID. For example, a cell ID may be obtained based on the signal pattern of a pilot signal. As another example, a cell ID may be obtained based on demodulation and decoding results of the P-SCH and the S-SCH. Meanwhile, when cell IDs are not grouped, the cell ID of the corresponding cell may be included in the S-SCH as an information item. In this case, the mobile station may obtain the cell ID by just demodulating and decoding the S-SCH.
However, if the above cell search method is applied to an inter-base-station synchronous system where signals from multiple cells are synchronized, S-SCHs transmitted from the cells using different sequences are demodulated and decoded based on channel estimates obtained based on P-SCHs transmitted from the cells using the same sequence. This in turn may degrade the transmission characteristics of the S-SCHs. Here, the transmission characteristics may include the time necessary for a cell search. Meanwhile, in an inter-base-station asynchronous system where signals from multiple cells are not synchronized, the above problem may not occur because the reception timings of P-SCH sequences transmitted from the cells are different from each other.
To prevent the above described characteristic degradation of S-SCHs in an inter-base-station synchronous system, a cell search method where plural sequences, for example, three or seven sequences, are used for P-SCHs is being studied (R1-062636, Cell Search Performance in Tightly Synchronized Network for E-UTRA). Also to prevent the above described characteristic degradation of S-SCHs in an inter-base-station synchronous system, a cell search method where P-SCHs are transmitted from plural cells at different transmission intervals has been proposed (R1-070428, Further analysis of initial cell search for Approach 1 and 2—single cell scenario). This method makes it possible to use P-SCHs received from plural cells at different reception timings for the demodulation and decoding of S-SCHs and thereby to prevent the characteristic degradation of the S-SCHs.
From the view point of cell design, it is preferable to use as many sequences as possible for P-SCHs in the method described in R1-062636 and as many different transmission intervals as possible for transmitting P-SCHs in the method described in R1-070428. If the number of sequences used for P-SCHs is small, the probability that the same sequence is used for P-SCHs of adjacent cells increases. Also, if the number of different transmission intervals for transmitting P-SCHs is small, the probability that P-SCHs of adjacent cells are transmitted at the same transmission interval increases. This in turn increases the probability of occurrence of characteristic degradation of S-SCHs in an inter-base-station synchronous system.
Meanwhile, the time necessary for a cell search, i.e., the transmission characteristics in a cell search, and the processing load of the mobile station in a cell search are incompatible with each other. Therefore, it is preferable to configure a system to enable the user to select whether to attach importance to the transmission characteristics in a cell search or to the processing load of the mobile station in a cell search by setting parameters or by changing operational methods.